Aluminum nitride body having graded metallurgy

ABSTRACT

Disclosed is an aluminum nitride body having graded metallurgy and a method for making such a body. The aluminum nitride body has at least one via and includes a first layer in direct contact with the aluminum nitride body and a second layer in direct contact with, and that completely encapsulates, the first layer. The first layer includes 30 to 60 volume percent aluminum nitride and 40 to 70 volume percent tungsten and/or molybdenum while the second layer includes 90 to 100 volume percent of tungsten and/or molybdenum and 0 to 10 volume percent of aluminum nitride.

RELATED APPLICATION

This application is a divisional of U.S. Pat. application Ser. No.08/361,351 filed on Dec. 21, 1994, pending and assigned to the assigneeof the present invention.

BACKGROUND OF THE INVENTION

This invention relates to an aluminum nitride body and the method ofproducing such a body and, more particularly, relates to an aluminumnitride body having features and vias which have a graded metallurgystructure and to a method for producing such an aluminum nitride body.

Aluminum nitride has been of interest recently for electronic packagingapplications because of its high thermal conductivity, thermal expansionmatching with silicon, low dielectric constant (8.5) and high electricalresistivity.

The present invention is particularly suitable for co-fired electronicpackages, also known as substrates. In one co-firing process, thealuminum nitride is formed into greensheets (comprised of aluminumnitride particles in an organic binder), vias are punched, metallizationpaste (comprised of metallic particles in an organic binder) is screenedor extruded onto the greensheets and into the vias, the greensheets arestacked and laminated to form a substrate in the green state, and thenthe green substrate is sintered to densify the aluminum nitride layersand the metallization. "Co-fired" means that the metallic paste issintered during the same sintering schedule as the aluminum nitridebody. The metallization for aluminum nitride substrates is typicallytungsten but may also be molybdenum or a mixture of tungsten andmolybdenum. In addition, instead of forming the aluminum nitride body byusing greensheets, dry pressing may be used to form the aluminum nitridebody.

The co-firing of aluminum nitride substrates with tungsten metallurgyrequires matching of the sintering behavior of aluminum nitride andtungsten. This can be achieved by introducing various sinteringadditives to the aluminum nitride and tungsten powders. Generally, thetungsten powder shows sintering onset at a lower temperature compared toaluminum nitride and hence the sintering additives added to tungstenretard the onset of tungsten sintering. The sintering additives may alsoreduce the final sintered density of tungsten, possibly resulting inporous tungsten. This type of metallurgy, though useful for internalmetallization, cannot be used to produce input/output (I/O) pads orsurface metallization due to its porous nature. Use of porousmetallization results in a non-hermetic substrate which is undesirablebecause any wet processing subsequent to the formation of the I/O padswill allow liquids to penetrate within the substrate which can degradethe manufacturability and performance of the substrate. Use of puretungsten powder that will sinter to high density (and therefore behermetic) for external metallization results in early sintering oftungsten, leading to debonding at the aluminum nitride/tungsteninterface before the aluminum nitride sinters to full density, givingvery low adhesion strength.

The solution to this dilemma involves addressing two problems that areinterdependent, namely, shrinkage matching of tungsten metallization tothat of aluminum nitride during co-firing by choosing the rightsintering additives and ensuring that fully dense tungsten resultsnotwithstanding the presence of sintering additives that retard thesintering of tungsten.

Thus far, this solution remains unfulfilled.

Okuno et al. U.S. Pat. Nos. 4,695,517 and 4,800,137, the disclosures ofwhich are incorporated by reference herein, have proposed a compositelayer on an aluminum nitride body in an effort to increase the bondstrength between the aluminum nitride body and the metallization. Okunoet al. propose overlapped layers on the aluminum nitride body oftungsten (or molybdenum) plus aluminum nitride followed by a layer oftungsten (or molybdenum). Okano et al. do not appear to appreciate thatthe first layer may be porous, thereby necessitating that it becompensated for lest a non-hermetic substrate should result. Even if thefirst layer of tungsten (or molybdenum) plus aluminum nitride isnonporous, it must be completely covered by the overlying layer oftungsten (or molybdenum) because any subsequent plating will not adherewell to the first layer, resulting in damage to the entire plated layer.A plated layer is usually disposed over the tungsten (or molybdenum)layer in order to facilitate I/O pin or wire bonding.

Sato et al. European Patent Application 0 574 956, the disclosure ofwhich is incorporated by reference herein, propose an intermediateco-fired layer of tungsten or molybdenum plus titanium nitride followedby metallization such as by nickel plating.

Harada et al. U.S. Pat. Nos. 4,980,239 and 5,096,749, the disclosures ofwhich are incorporated by reference herein, discloses a composite layerstructure on an aluminum nitride substrate comprising sequential thinfilm layers of titanium, tungsten or molybdenum, and nickel. Thereafter,the composite structure is heated to a temperature high enough to causea reaction between the titanium layer and the aluminum nitride body,resulting in an intermediate layer of aluminum titanium nitride whichimproves the adhesion of the metallization.

Iio et al. U.S. Pat. Nos. 4,840,853 and 4,892,703, the disclosures ofwhich are incorporated by reference herein, propose an aluminum nitridebody having an intermediate layer comprising aluminum, nitrogen andoxygen, and a metallized layer of tungsten or molybdenum. Theintermediate layer is necessary to increase the bonding strength of themetallized layer. It is not clear from the description how thisstructure is formed.

The internal vias of the substrate present a different problem. Puretungsten or molybdenum will not adhere well to the walls of the vias.So-called "rattling vias"may be obtained where the metallic via floatswithin the via opening which may result in cracking around vias.Accordingly, the present inventors have recognized that a gradedmetallurgy may be useful for the internal vias.

Knickerbocker et al. U.S. Pat. 5,260,519, the disclosure of which isincorporated by reference herein, discloses a multiple layer/viastructure wherein the composition of the vias indifferent layers isgraded from metallic to a mixture of ceramic and metal. There is noteaching of grading the via composition in a single via opening.

Dolhert, et al., U.S. Pat. 5,200,249, the disclosure of which isincorporated by reference herein, discloses an hermetic via compositionfor an aluminum nitride substrate consisting of a mixture of aluminum,nitride and tungsten or molybdenum.

Panicker, et al., U.S. Pat. 4,942,076, the disclosure of which isincorporated by reference herein, discloses a composite via compositionfor an alumina substrate. The substrate is sintered with tungsten pastewhich forms a porous via. Copper is later infiltrated into the poroustungsten.

In view of the above attempts at composite and graded structures, it isa purpose of the present invention to have a metallurgical structurewith good adhesion strength to accommodate the bonding of I/O pins andwire bonds.

It is another purpose of the present invention to have a metallurgicalstructure that is hermetic with respect to the ambient environment.

It is yet another purpose of the present invention to have ametallurgical structure that is particularly useful for making compositevias.

These and other purposes of the invention will become more apparentafter referring to the following description in conjunction with theaccompanying drawings.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is proposed analuminum nitride sintered body having graded metallurgy comprising:

an aluminum nitride sintered body having at least one via therein; and

graded metallurgy on said sintered body and in direct contact with saidat least one via, said graded metallurgy comprising a first layer ofmetallurgy in direct contact with said sintered body and a second layerof metallurgy in direct contact with, and that completely encapsulates,said first layer of metallurgy;

said first layer of metallurgy comprising 30 to 60 volume percentaluminum nitride and 40 to 70 volume percent of a metal selected fromthe group consisting of tungsten, molybdenum and mixtures thereof;

said second layer of metallurgy comprising 90 to 100 volume percent of ametal selected from the group consisting of tungsten, molybdenum andmixtures thereof and 0 to 10 volume percent of aluminum nitride.

According to a second aspect of the invention, there is proposed analuminum nitride sintered body having graded metallurgy comprising:

an aluminum nitride sintered body having at least one via therein;

said at least one via being filled with graded metallurgy, said gradedmetallurgy comprising a first layer of metallurgy adhered to the wallsof said at least one via but leaving a central portion of said at leastone via devoid of said first layer of metallurgy and a second layer ofmetallurgy filling the central portion of said at least one via;

said first layer of metallurgy comprising 30 to 60 volume percentaluminum nitride and 40 to 70 volume percent of a metal selected fromthe group consisting of tungsten, molybdenum and mixtures thereof;

said second layer of metallurgy comprising 90 to 100 volume percent of ametal selected from the group consisting of tungsten, molybdenum andmixtures thereof and 0 to 10 volume percent of aluminum nitride.

According to a third aspect of the invention, there is proposed a methodfor producing an aluminum nitride sintered body having graded metallurgycomprising the steps of:

providing an aluminum nitride unsintered body having at least one viatherein;

forming a first layer of metallurgy paste proximate to said at least onevia and in direct contact with said aluminum nitride body, said firstlayer of metallurgy paste comprising, based on the solids content ofsaid paste, 30 to 60 volume percent aluminum nitride and 40 to 70 volumepercent of a metal selected from the group consisting of tungsten,molybdenum and mixtures thereof;

forming a second layer of metallurgy paste in direct contact with, andthat completely encapsulates, said first layer of metallurgy paste, saidsecond layer of metallurgy paste, based on the solids content of saidpaste, comprising 90 to 100 volume percent of a metal selected from thegroup consisting of tungsten, molybdenum and mixtures thereof and 0 to10 volume percent of aluminum nitride; and

sintering said aluminum nitride body and said first and second layers ofmetallurgy paste at a predetermined time and temperature to form a fullydense aluminum nitride sintered body having first and second layers ofgraded metallurgy wherein said second layer of metallurgy completelyencapsulates said first layer of metallurgy and wherein said gradedmetallurgy is in contact with said at least one via.

According to a fourth aspect of the invention, there is proposed amethod for producing an aluminum nitride sintered body having gradedmetallurgy comprising:

providing an aluminum nitride sintered body having at least one viatherein;

filling said at least one via with graded metallurgy paste, said gradedmetallurgy paste comprising a first layer of metallurgy paste adhered tothe walls of said at least one via but leaving a central portion of saidat least one via devoid of said first layer of metallurgy paste and asecond layer of metallurgy paste filling the central portion of said atleast one via;

said first layer of metallurgy paste comprising 30 to 60 volume percentaluminum nitride and 40 to 70 volume percent of a metal selected fromthe group consisting of tungsten, molybdenum and mixtures thereof;

said second layer of metallurgy paste comprising 90 to 100 volumepercent of a metal selected from the group consisting of tungsten,molybdenum and mixtures thereof and 0 to 10 volume percent of aluminumnitride; and

sintering said aluminum nitride body and first and second layers ofmetallurgy paste at a predetermined time and temperature to form a fullydense aluminum nitride sintered body having at least one via thereinwith first and second layers of graded metallurgy.

According to a fifth aspect of the invention, there is proposed amultilayered aluminum nitride sintered body comprising:

at least two sintered layers of aluminum nitride;

at least one metallurgical feature situated between said at least twosintered layers of aluminum nitride, said metallurgical featurecomprising 30 to 60 volume percent aluminum nitride and 40 to 70 volumepercent of a metal selected from the group consisting of tungsten andmolybdenum and mixtures thereof.

According to a sixth aspect of the invention, there is proposed aco-fired aluminum nitride sintered body having adherent gradedmetallurgy comprising:

an aluminum nitride sintered body; and

graded metallurgy on, and in direct contact with, said sintered body,said graded metallurgy comprising a first layer of metallurgy in directcontact with said sintered body and a second layer of metallurgy indirect contact with, and that completely encapsulates, said first layerof metallurgy;

said first layer of metallurgy comprising 30 to 60 volume percentaluminum nitride and 40 to 70 volume percent of a metal selected fromthe group consisting of tungsten, molybdenum and mixtures thereof;

said second layer of metallurgy comprising 90 to 100 volume percent of ametal selected from the group consisting of tungsten, molybdenum andmixtures thereof and 0 to 10 volume percent of aluminum nitride.

According to a seventh aspect of the invention, there is proposed amethod for producing a co-fired aluminum nitride sintered body havingadherent graded metallurgy comprising the steps of:

providing an aluminum nitride unsintered body;

forming a first layer of metallurgy paste in direct contact with saidaluminum nitride body, said first layer of metallurgy paste comprising,based on the solids content of said paste, 30 to 60 volume percentaluminum nitride and 40 to 70 volume percent of a metal selected fromthe group consisting of tungsten, molybdenum and mixtures thereof;

forming a second layer of metallurgy paste in direct contact with, andthat completely encapsulates, said first layer of metallurgy paste, saidsecond layer of metallurgy paste, based on the solids content of saidpaste, comprising 90 to 100 volume percent of a metal selected from thegroup consisting of tungsten, molybdenum, and mixtures thereof, and 0 to10 volume percent of aluminum nitride; and

sintering said aluminum nitride body and said first and second layers ofmetallurgy paste at a predetermined time and temperature to form a fullydense co-fired aluminum nitride sintered body having first and secondlayers of graded metallurgy wherein said second layer of metallurgycompletely encapsulates said first layer of metallurgy and wherein saidgraded metallurgy is in direct contact with, and adherent to, saidaluminum nitride sintered body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of the inventionwherein an aluminum nitride sintered body has graded metallurgy for anI/O pad and the bottom layer is in contact with the via.

FIG. 2 is a cross-sectional view of another embodiment of the inventionwherein an aluminum nitride sintered body has graded metallurgy if or anI/O pad and the top layer is in contact with the via.

FIG. 3 is a cross-sectional view of another embodiment of the inventionwherein two different metallurgies are provided in a single via in orderto provide better adhesion and conductivity of the via.

FIG. 4 is a cross-sectional view of a further embodiment of theinvention wherein a metallurgical feature of the composition accordingto the present invention is provided between two layers of sinteredaluminum nitride.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures in more detail, and particularly referring toFIG. 1, disclosed according to the invention is an aluminum nitride bodyhaving graded metallurgy, generally indicated by 10 in FIG. 1. Aluminumnitride sintered body 12 has at least one via 14 in the body. Thecomposition of this particular via 14 is not important to this aspect ofthe invention. Typically however, via 14 usually comprises tungsten,molybdenum or mixtures of tungsten and molybdenum and may also includecertain sintering additives, such as calcia, alumina, or yttria, toachieve densification of the via.

Situated on aluminum nitride sintered body 12 is graded metallurgy 16which is also positioned to be in contact with via 14. As shown in FIG.1, graded metallurgy 16 comprises a first layer of metallurgy 18 whichis in direct contact with the aluminum nitride sintered body 12 and via14 and a second layer of metallurgy 20 that is in direct contact withthe first layer of metallurgy 18. Second layer of metallurgy 20 alsocompletely encapsulates first layer of metallurgy 18.

The first layer of metallurgy comprises 30 to 60 volume percent aluminumnitride and 40 to 70 volume percent of tungsten or molybdenum or amixture of tungsten and molybdenum, preferably tungsten. It is believedthat a minimum of 30 volume percent of aluminum nitride is necessary forgood bonding. This belief is based on a metallographic evaluation of theinterface between the aluminum nitride sintered body and the first layerof metallurgy 18 where an abrupt interface is shown. Lesser amounts ofaluminum nitride in the first layer of metallurgy 18 would probablyreduce the integrity of this interface. On the other hand, ametallographic evaluation of the interface for 60 volume percentaluminum nitride in the first layer of metallurgy 18 showed a verydiffuse interface, thereby indicating the likelihood of good bonding.However, more than 60 volume percent aluminum nitride in the first layerof metallurgy 18 is discouraged as conductivity and adhesion to thesecond layer of metallurgy 20 become impaired. The most preferred amountof aluminum nitride is about 55 volume percent which provides the rightbalance of adhesion and conductivity. In addition, the first layer ofmetallurgy may contain sintering aids such as yttria or calcia to assistin densification, if desired.

The second layer of metallurgy 20 comprises 90 to 100 volume percent oftungsten or molybdenum or a mixture of tungsten and molybdenum, withtungsten being preferred, and 0 to 10 volume percent aluminum nitride.Small amounts of aluminum nitride may be added to the metal tomarginally increase the adhesion of the second layer of metallurgy 20 tothe first layer of metallurgy 18. It is preferred that the second layerof metallurgy 20 be 100 volume percent tungsten. In any event, thesecond layer of metallurgy 20 sinters to a substantially pore-free statewhich is hermetic.

The graded metallurgy 16 is typically used for I/O or wire bond pads. Assuch, the graded metallurgy 16 may be plated with a metal such as nickelto facilitate soldering or brazing of the wire or pins to the bond pad.In its most general aspect, the present invention may be used to formadherent, graded metallurgy on a sintered aluminum nitride sintered bodyfor seal bands, lands for attachment of lead frames and surfacemetallization. In the latter uses, a via may not be present orelectrically connected tot he graded metallurgy. Even here though, thegraded metallurgy 16 will usually be plated.

It is important to the present invention that the first layer ofmetallurgy 18 is completely encapsulated by the second layer ofmetallurgy 20. There are two reasons for this. The first reason is thatfirst layer of metallurgy may be porous which can occur if sinteringaids are not added to the first layer of metallurgy 18 during sintering.During further processing of the sintered aluminum nitride body, whichcan include wet processing for plating, moisture or liquids can seepinto the first layer of metallurgy if it is porous and then further seepinto the rest of the substrate since the via is often porous. Thispresents a manufacturability as well as a reliability problem. Thesecond reason (whether or not the first layer is porous) is that thegraded metallurgy is typically plated to enhance solderability orbrazeability. Plating will not adhere well to the first layer ofmetallurgy 18. And, if even a small part of the first layer ofmetallurgy is exposed to the plating, the integrity of the entire platedlayer can be adversely impacted. Thus, second layer of metallurgy 20must completely encapsulate first layer of metallurgy 18.

Referring now to FIG. 2, there is an alternative embodiment of thepresent invention. As in FIG. 1, there is an aluminum nitride sinteredbody having graded metallurgy, generally indicated by 10'. Gradedmetallurgy 16' comprises first layer of metallurgy 18' and second layerof metallurgy 20'. First layer of metallurgy 18' is in direct contactwith aluminum nitride sintered body 12 but in this particular embodimentdoes not contact via 14. First layer of metallurgy 18' may be picturedas being donut-shaped with via 14 being in the "hole" of the donut.Then, second layer of metallurgy 20' is positioned over, and is indirect contact with, via 14. Second layer of metallurgy 20' alsocompletely encapsulates first layer of metallurgy 18' for the samereasons as stated before. The advantage to this embodiment of theinvention is that higher conductivity second layer of metallurgy 20' isin direct contact with via 14 and yet adhesion of the second layer ofmetallurgy 20' is still provided by the first layer of metallurgy 18'.

The embodiments of FIGS. 1 and 2 may be made by the following procedure.An unsintered aluminum nitride body is provided, which may be monolithicor multilayered. Then, a first layer of metallurgy paste is deposited onthe unsintered aluminum nitride body and proximate to the unsinteredvia. (When used for seal bands and the like, the graded metallurgy maybe formed away from the via. In this case, the requirement is for thegraded metallurgy to be formed directly on the unsintered aluminumnitride body.) As is well known in the art, the metallurgy pastecomprises metallic particles, a binder, solvent, plasticizer, and otherorganic additives. After deposition of the metallurgy paste and followedby sintering, the solvent and organic materials are removed from thepaste and the remaining metallic particles densify into a layer ofmetallurgy. As used herein, the term "proximate" shall mean on or nearthe via. Thus, the first layer of metallurgy paste may be depositeddirectly on the via, to result in the structure shown in FIG. 1, oraround the via (but not in direct contact with the via), to result inthe structure as shown in FIG. 2. The first layer of metallurgy pastecomprises 30 to 60 volume percent aluminum nitride and 40 to 70 volumepercent of tungsten or molybdenum or a mixture-of tungsten andmolybdenum, preferably tungsten, based on the solids content of thepaste, that is, excluding the solvent, binder, and other organicadditives.

In addition, the first layer of metallurgy paste may contain sinteringaids, as mentioned earlier. Some of these sintering aids may include,for example, yttria and calcia, as is well known to those skilled in theart. The preferred sintering aids are either a mixture ofcalcia-alumina-boria as disclosed in Duncombe et al. U.S. patentapplication Ser. No. 08/173,293, now U.S. Pat. No. 5,482,903, or yttriaplus calcia-alumina-boria as disclosed in Harris et al. U.S. patentapplication Ser. No. 08/172,032, now U.S. Pat. No. 5,424,261, thedisclosures of both of which are incorporated by reference herein.

Second layer of metallurgy paste is then deposited over, and completelyencapsulates, the first layer of metallurgy paste. The composition ofthe second layer of metallurgy paste comprises 90 to 100 volume percentof tungsten or molybdenum or a mixture of tungsten and molybdenum,preferably tungsten, and 0 to 10 volume percent of aluminum nitride,based on the solids content of the paste. As before, these volumefractions exclude the solvent, binder and other organic additives.Depending on whether the FIG. 1 or FIG. 2 embodiment is to be formed,the second layer of metallurgy paste may or may not be in direct contactwith the via.

Then, the unsintered aluminum nitride body and the first and secondlayers of metallurgy paste are sintered (co-fired) at a predeterminedtime and temperature to form a fully dense aluminum nitride sinteredbody with first and second layers of adherent metallurgy. The gradedmetallurgy so formed is positioned so as to be in direct contact withvia 14 of sintered aluminum nitride body 12.

The sintering schedule may be conveniently chosen by those skilled inthe art to achieve the desired microstructure and physical properties.Generally speaking, the aluminum nitride body may be sintered in aconventional furnace so long as there is a protective atmosphere. Apreferred atmosphere is forming gas which is a mixture of nitrogen andhydrogen gases. The temperature is ramped up to about 600 degreesCentigrade to pyrolyze the binder. Then, the temperature is slowlyramped up to the sintering temperature of about 1550 to 1650 degreesCentigrade and held there for a period of time to accomplish binderburnoff and densification. Finally, the temperature is ramped down toroom temperature.

Referring now to FIG. 3, there is shown another embodiment of thepresent invention. An aluminum nitride sintered body 30 contains atleast one via 38 which is filled with graded metallurgy, generallyindicated by 32. First layer of metallurgy 34 adheres to the walls ofthe via 38 but leaves an open central portion of the via 38 which isfilled by second layer of metallurgy 36. First and second layers ofmetallurgy 34, 36 together make up graded metallurgy 32. Thecompositions of the first and second layers of metallurgy 34, 36 are thesame as those discussed above.

In the prior art, where the via metallization did not adhere well to thewalls of the via, the vias could "rattle around" or move around withinthe via, which could lead to cracking around the via. Accordingly, thepresent inventors have proposed a graded metallurgy for the via whichhas several advantages. The first is that rattling vias are prevented.The second is that the first layer of metallurgy provides good adhesionto the walls of the via. And the third is that the second layer ofmetallurgy, while being tightly adhered to the first layer ofmetallurgy, provides a highly conductive core for the transmittance ofpower and signals.

The embodiment of FIG. 3 may be made by filling the via of an unsinteredaluminum nitride body with graded metallurgy paste. This can beaccomplished by first applying a metallurgy paste to the viacorresponding to the composition of the first layer of metallurgy paste.A low viscosity paste is used for this purpose. The metallurgy paste isthen allowed to dry, with the result that the metallurgy paste shrinksin volume and adheres only to the walls of the via. Thus, a centralportion of the via is left without or devoid of paste. Then, a secondmetallurgy paste, corresponding to the composition of the second layerof metallurgy paste is applied to the via so as to fill the centralportion of the via left vacant by the first layer of metallurgy paste.Then, the unsintered aluminum nitride body and via with gradedmetallurgy paste is sintered as described above to resulting thestructure as shown in FIG. 3.

Referring now to FIG. 4, there is disclosed the last embodiment of thepresent invention. There is a multilayered aluminum nitride sinteredbody, generally indicated by 40. For ease of illustration, the variouslayers of the sintered body 40 are shown separated. The multilayeredaluminum nitride sintered body 40 comprises at least two sintered layers42, 44 of aluminum nitride and at least one metallurgical feature 46situated between the two sintered layers 42, 44 of aluminum nitride. Thecomposition of the metallurgical feature 46 comprises 30 to 60 volumepercent aluminum nitride and 40 to 70 volume percent of tungsten ormolybdenum or a mixture of tungsten and molybdenum, preferably tungsten.The metallurgical feature 46 may be a wiring line to connect via 50 insintered layer 44 to via 48 in sintered layer 42. In addition,metallurgical feature 46 may be used in conjunction with vias havinggraded metallurgy as shown in FIG. 3. Alternatively, metallurgicalfeature 46 may form a power plane or ground plane. Power and groundplanes contain a substantial quantity of metallization in a single layerand can suffer from delamination due to poor adhesion to the adjoiningceramic layers. By making the power and/or ground planes from thecomposition of the metallurgical feature 46, better adhesion can resultwithout a significant decrease in electrical performance of the powerand/or ground planes.

EXAMPLES

Peel Test Results:

Six aluminum nitride multilayer substrates were prepared. Each of thesubstrates consisted of 3 weight percent yttria (1-5μ particle size), 1weight percent calcia-alumina-boria glass (4-8μ particle size),remainder aluminum nitride (1-1.5μ aggregate particle size). Paste wasscreened on to form nine pads on each substrate. Four of the substratesreceived first and second layers of tungsten (1-3μ particle size)metallurgical paste, denoted in the Table as pastes I and II,respectively. Two of the substrates received only:the second layer ofmetallurgical paste (paste II). Generally speaking, the smaller theparticle sizes for the yttria, calcia-alumina-boria glass, aluminumnitride and metallic particles are preferred for enhanced densification.

After undergoing binder burnoff, the substrates were sintered at 1600degrees Centigrade for 10-28 hours. Wires were then soldered to the padswith Cu/Sn solder (66 weight percent copper; 34 weight percent tin) andpulled at an angle of 90 degrees with respect to the pads.

The average peeling load for those substrates containing first andsecond layers of metallurgy (pastes I and II) was 3.6 to 4.2 pounds. Allfailures occurred between the solder and pad. There were no failsbetween the pad and ceramic.

The average peeling load for those substrates containing only the secondlayer of metallurgy (paste II) was 1.1 to 1.4 pounds. All the failuresoccurred at the ceramic/pad interface, which is extremely undesirable.

                  TABLE                                                           ______________________________________                                        METALLURGY PASTES COMPOSITION (Volume %)                                      COMPONENT        PASTE I  PASTE II                                            ______________________________________                                        SOLIDS                    31                                                  Tungsten         15                                                           Aluminum Nitride 18                                                           Cab* Glass       <.5                                                          Yttria           <.5                                                          Total Solvent    51       51                                                  Total Non-Solvent                                                                              15       18                                                  Organics                                                                      ______________________________________                                         *calcia-alumina-boria                                                    

I/O PIN PULL TESTS RESULTS:

An additional 6 substrates were prepared in the manner as describedabove except that 168 I/O pads were screened on each substrate. As wasdone above, the pads on four of the substrates were screened with firstand second metallurgical pastes (pastes I and II) while the pads on twoof the substrates were screened with second metallurgical paste (pasteII) only.

After sintering as above, I/O pins were brazed to the pads with Cu/Agbraze. The pins were then pulled at an angle of 70 degrees with respectto the pads.

The results indicate that the substrates with first and second layers ofmetallurgy experienced no pad failures. All failures were pin shankfails at 18 pounds. The substrates with only the second layer ofmetallurgy failed at 5 to 18 pounds; 1-2% of the pads failed through paddelamination.

ZYGLO PENETRATION RESULTS:

Several of the substrates above were exposed to Zyglo penetrant and thensectioned. Those substrates having first and second layers of metallurgyexhibited no Zyglo penetration while those substrates having only thesecond layer of metallurgy exhibited Zyglo penetration at theceramic/pad interface indicating partial delamination sintering.

These results indicate the advantages of using graded metallurgy ratherjust a single layer of tungsten or molybdenum metallurgy.

It will be apparent to those skilled in the art having regard to thisdisclosure that other modifications of this invention beyond thoseembodiments specifically described here may be made without departingfrom the spirit of the invention. Accordingly, such modifications areconsidered within the scope of the invention as limited solely by theappended claims.

What is claimed is:
 1. A method for producing an aluminum nitridesintered body having graded metallurgy comprising:providing an aluminumnitride sintered body having at least one via therein; filling said atleast one via with graded metallurgy paste, said graded metallurgy pastecomprising a first layer of metallurgy paste adhered to the walls ofsaid at least one via but leaving a central portion of said at least onevia devoid of said first layer of metallurgy paste and a second layer ofmetallurgy paste filling the central portion of said at least onevia;said first layer of metallurgy paste comprising 30 to 60 volumepercent aluminum nitride and 40 to 70 volume percent of a metal selectedfrom the group consisting of tungsten, molybdenum and mixtures thereof;said second layer of metallurgy paste comprising 90 to 100 volumepercent of a metal selected from the group consisting of tungsten,molybdenum and mixtures thereof and 0 to 10 volume percent of aluminumnitride; and sintering said aluminum nitride body and first and secondlayers of metallurgy paste at a predetermined time and temperature toform a fully dense aluminum nitride sintered body having at least onevia therein with first and second layers of graded metallurgy.
 2. Themethod of claim 1 wherein the step of filling comprises the stepsof:applying said first layer of metallurgy paste to said at least onevia so as to fill said via; drying said first layer of metallurgy pastewithin said via, said drying causing the first layer of metallurgy pasteto shrink in volume and adhere only to the walls of said via, therebyleaving the central portion of said at least one via devoid of saidfirst layer of metallurgy; and applying said second layer of metallurgypaste to said at least one via and over said first layer of metallurgypaste so as to fill said via.
 3. The method of claim 1 wherein saidfirst layer of metallurgy further comprises a sintering aid.
 4. Themethod of claim 1 wherein said metal in said first and second layers istungsten.
 5. The method of claim 1 wherein said second layer ofmetallurgy is 100 volume percent metal.
 6. The method of claim 5 whereinsaid metal is tungsten.